1. Field of Invention
This invention pertains to paper web coating systems and more particularly to a coating system for applying a coating composition onto a paper web.
2. Description of the Prior Art
According to a known method a moving web is treated with liquid by passing the web tightly over an opening in a container connected to a liquid supply and to a vacuum source so that the vacuum is maintained in said container and the liquid is drawn by said vacuum up over said opening, thus reaching the web. Attempts have also been made to arrange a further opening, which could be divided into one or more rows of holes, in this known container and furthermore a horizontal rod bas been placed longitudinally across said row of holes. This arrangement has been used to remove excess liquid from the web when passing said rod due to the vacuum in said container. However this removed excess liquid is recirculated to the lower part of the container containing fresh liquid to be supplied to the web.
This known device may be used for treating paper webs with water, surface glue in the form of a starch solution and other liquids having low viscosity. The advantage of this known system is that it is compact and that if the web breaks because wetting is performed under vacuum, only a small quantity of the liquid will be spilled.
In producing coated paper for the use as printing paper at least one side of the paper web is coated with a suspension containing pigments, for example clay in water. Those suspensions for coating compositions genrally also contain a number of chemical additives, such as binders, dispersing agent etc. The water content in that coating composition is partly absorbed during the coating of the web and simultaneously a filter cake of pigment is built up on the surface of the web. After this coating operation the web is passed through suitable drying means so that the absorbed water is caused to evaporate either entirely or partially.
This necessary evaporation step not only affects the ultimate costs of the finished product due to the very nature of the coating process, but also requires that the coating be performed with a coating composition which is as dry as possible. It has recently been established that blade-coating systems are highly recommendable for coating compositions having a high dryness content. Such blade-coating systems comprises a thin, flexible blade arranged to press the web against a backing surface, such as a rotating roller, and thereby smoothing the applied coating composition evenly over the web and at the same time remove the excess coating composition from the web. These known techniques also have the advantage of enabling very high speed to be used for such systems because, due to the high dryness content of the composition, the relatively minor quantity of water can be evaporated without using large drying equipment. It has been found that coating compositions with dryness contents of between 55-60 percent by weight of dry substance could be used in such blade coating systems, but even higher dryness contents are mainly used.